A Critical Role for Syk in Signal Transduction and Phagocytosis Mediated by Fcγ Receptors on Macrophages

Abstract

Receptors on macrophages for the Fc region of IgG (FcγR) mediate a number of responses important for host immunity. Signaling events necessary for these responses are likely initiated by the activation of Src-family and Syk-family tyrosine kinases after FcγR cross-linking. Macrophages derived from Syk-deficient (Syk−) mice were defective in phagocytosis of particles bound by FcγRs, as well as in many FcγR-induced signaling events, including tyrosine phosphorylation of a number of cellular substrates and activation of MAP kinases. In contrast, Syk− macrophages exhibited normal responses to another potent macrophage stimulus, lipopolysaccharide. Phagocytosis of latex beads and Escherichia coli bacteria was also not affected. Syk− macrophages exhibited formation of polymerized actin structures opposing particles bound to the cells by FcγRs (actin cups), but failed to proceed to internalization. Interestingly, inhibitors of phosphatidylinositol 3-kinase also blocked FcγR-mediated phagocytosis at this stage. Thus, PI 3-kinase may participate in a Syk-dependent signaling pathway critical for FcγR-mediated phagocytosis. Macrophages derived from mice deficient for the three members of the Src-family of kinases expressed in these cells, Hck, Fgr, and Lyn, exhibited poor Syk activation upon FcγR engagement, accompanied by a delay in FcγR-mediated phagocytosis. These observations demonstrate that Syk is critical for FcγR-mediated phagocytosis, as well as for signal transduction in macrophages. Additionally, our findings provide evidence to support a model of sequential tyrosine kinase activation by FcγR's analogous to models of signaling by the B and T cell antigen receptors.

Protein-Tyrosine Kinase Syk Expressed in Human Nasal Fibroblasts and Its Effect on RANTES Production

“…Syk is upstream in mediating high affinity IgE receptor (Fc⑀RI) (17), Fc receptor ␥-chain RII (19,47), IL-2R (48), and CD40 (49). Recently, Toll-like receptor (TLR)-2 or TLR-4 has been proven to mediate LPS-induced cellular signaling (50 -52).…”

“…Syk, one of the well-known tyrosine kinases, is widely expressed and plays an important role in intracellular signal transduction in hemopoietic cells: B cells (15,16), mast cells (17), platelets (18), macrophages (19), monocytes (20,21), basophils (22), neutrophils (23), T cells (24), NK cells (25), eosinophils (26), and erythrocytes (27). However, there has been no study that demonstrates the expression of Syk in human nonhemopoietic cells.…”

Redundant role for Zap70 in B cell development and activation

“…Bone marrow, spleen and lymph node cell suspensions were resuspended in ACK lysis buffer (155 mM NH 4 Cl, 10 mM KHCO 3 , 100 lM EDTA) for 2 min at room temperature to lyse red blood cells. The cells were then treated with anti-CD16/32 (24G2) antibody to block nonspecific antibody binding, before adding pre-titered antibodies to cell surface antigens, followed by secondary reagents where required.…”

“…The cells were then treated with anti-CD16/32 (24G2) antibody to block nonspecific antibody binding, before adding pre-titered antibodies to cell surface antigens, followed by secondary reagents where required. For intracellular staining of Zap70, cells were fixed in 3% paraformaldehyde after the surface stain, autofluorescence was quenched by treatment with 50 mM NH 4 Cl, cells were permeabilized using 0.1% NP-40, 150 mM NaCl, 50 mM Tris-Cl pH 7.5 for 3 min and then incubated with antiZap70-PE antibody overnight. Intracellular staining for Syk was performed using the Fix&Perm Kit (Caltag) according to the manufacturer's protocol.…”

“…This binding results in the phosphorylation and activation of Syk and subsequent signaling to numerous downstream pathways. The importance of Syk in transducing signals from these receptors has been demonstrated in B cells, mast cells and macrophages deficient in Syk [2][3][4][5][6].…”